This invention is directed to a gun for dispensing a plural component system which includes a valve carrier movable within the gun such that both the component control valves and an air control valve are movable in response to the valve carrier. In one position of the valve carrier the components are dispensed through a mixing chamber and in another position of the valve carrier the mixing chamber is purged of any residual components by dispensing air into the mixing chamber.
For dispensing polyurethane foam systems and other plural component systems wherein two or more components are mixed immediately prior to dispensing, several dispensing guns are known. These guns have been developed because the components of the respective plural component system polymerize very rapidly upon mixing. Upon polymerization the components set up into masses which cannot be conveniently dispensed. Any attempt to mixing the components prior to dispensing inevitably leads to clogging or plugging of the dispensing system.
In any dispensing system for plural components it is necessary to mix these components somewhere in the dispensing system. This requires some form of mixing chamber to always be included in the dispensing system. Upon cessation of use of the plural components some material remains in the mixing chamber and if not removed will polymerize in the mixing chamber into a fused mass which must be mechanically removed. This of course is impractical to do. Further, in those systems wherein the dispensing unit is continually being cycled off, it is of course impractical to have to repeatedly clean the mixing chamber.
In order to circumvent these problems spray guns have been devised which provide for cleaning of the mixing chamber with either a mechanical plunger or a blast of pressurized air each time the stream of the individual members of the plural component system is interrupted. One such system relies on a piston which is machined to fit the walls of the mixing chamber and mechanically forces out any remaining portions of a plural component system upon returning the trigger of the spray gun to an off position. Unfortunately, the utility of this system requires maintaining of close tolerances between the mixing chamber and the cleansing piston. This system deteriorates with age and wear.
U.S. Pat. Nos. 3,799,403 and 3,873,023 describe spray gun type apparatuses which utilize an air blast to purge their mixing chambers. Both of these utilize a movable mixing chamber having a T-shaped passage located therein. The individual components of the plural component system are injected into the arms at the top of the T and they exit out of the base of the T. In both of these, the mixing chamber moves between a position wherein the components of the plural component system are injected into the arms of the T and a position wherein air is injected into the arms of the T. One of the patents describes a spray gun wherein the mixing chamber moves radially between these two positions and in the other the mixing chamber moves axially. In both of these, since the mixing chamber in fact moves either some machining to close tolerances of these areas of the mixing chamber wherein the openings of the arms are located and those areas of the housing wherein both the air and the plural components exit into the arms of the mixing chamber must be done or seals must be used in these areas. If the parts are machined, the machine parts of course are also susceptible to wear and any leakage between the movable mixing chambers and other components of the housing require disassembly of the unit to effect cleaning. If seals are used, the seals are continuously exposed to abrasion by the edges formed by the openings of the passageways moving across them each time the mixing chamber moves. This of course leads to early failure of the seals.
In any spray gun or similar device wherein the individual components of a plural component system are injected into passageways which are directly opposed to each other such as the passageways in the arms of the T's of the mixing chambers of the above noted patent, should the nozzle of the device become clogged one individual component can be injected into or flow into the passageway of the other components and thus mix with the other components within its passageway. Since the nozzle is clogged once mixing has been effected polymerization quickly follows and these passageways become clogged. Further, since these passageways are located in movable mixing chambers the length of the passageways is restricted by the size of the mixing chamber. Thus, the passageways are inevitably quite short. This makes it easier for one component to contaminate not only the passageway of the other components within the mixing chamber, but also the passageway of the other components within the housing and even into the supply lines.
In view of these problems with the prior art devices it is evident there exists a need for a dispensing system for plural component systems which utilize an air purge of the mixing chamber thus eliminating the need for high tolerances required in mechanical purge systems, but does not rely on a movable mixing chamber thus eliminating the need for close tolerances between the mixing chamber and the housing and allowing for longer component supply ducts or passageways. Further, since different plural component systems are sprayed under different pressure, the mechanics of the dispensing system must be adaptable to handle different pressures and different plural component systems.